1. Field of the Invention
The present invention relates to a device for checking the position and the orientation of a mirror.
When a movable mirror is used in an optical device placed in a cluttered environment, and when the image reflected by this mirror has to be positioned with high precision, it may be difficult or even impossible to use standard image pick-up means (such as sensors or path-changing mirrors) to give a value of the position and orientation of this mirror to its positional servocontrol device.
This is, in particular, the case with a head-up collimator whose combiner mirror is mobile. A head-up collimator has synthetic images and various items of information superimposed on the outside scene in an aircraft pilot's or helicopter pilot's field of view. All these information elements are used for piloting and must be placed with precision in relation to the axes of the aircraft.
The optical system of such a collimator is generally constituted by a collimation optical system and a combiner mirror. In the case of a collimator for transport aircraft, the combiner mirror is generally movable so that it can be stowed away during the stages in flight when the collimator is not used. It is vital, for reasons of flight safety, to be sure of the orientation and position of this mirror when the collimator is used.
2. Description of the Prior Art
Two types of optical systems for collimators are known:
in a first type of system, the combiner mirror is a plane mirror and its only role is to combine or mix synthetic images with the outside scene. In this case, it is only the orientation of the mirror that is important for the precision of the superimposition; PA1 in a second type of system, the combiner mirror is not plane and carries out both the collimation of the synthetic image and its combining with the outside scene. In this case, its orientation and its position affect the precision of the superimposition.
There is a system, known from the U.S. Pat. No. 4,775,218, for detecting the orientation of a head-up mirror based on the principle of self-collimation. This system enables the measurement of the orientation of a combiner mirror but does not give its position. Furthermore, this system uses an ancillary self-collimation mirror. The measurement is therefore not done on the combiner mirror itself.
There is a device, known from the French patent No. 92 14767, for checking the position of a mirror of a head-up visor. This device makes use of an optoelectronic four-quadrant diode-based detector detecting an image dot generated by the collimator itself. This device performs more efficiently than the above-mentioned device because it enables comprehensive control over the entire image-generating electro-optical channel, i.e. not only the right position of the combiner mirror but also the entire electronic part and image source producing the images. However, this device makes it necessary to position the sensor physically in the beam of light rays emerging from the collimator while, at the same time, not obstructing the view of the images. Now, either it is the case that the placing of a sensor or of a path-changing mirror in a device such as a head-up visor greatly hampers vision or else this placing cannot be done.